The performance of a grinding tool is determined mainly by the constituent materials used to prepare the tool. As an example, the grinding action and tool life of a vitrified grinding wheel are controlled primarily by the mount of abrasive and bond present, as well as the degree of porosity. For a given amount of abrasive, low porosity and high bond content result in hard action and long tool life. Conversely, high porosity and low bond content result in "softer" action, i.e., lower grinding power, and comparatively shorter tool life. The final porosity in conventional, cold-pressed grinding tools is controlled by varying the bond/abrasive ratio, as well as the density achieved in the cold-pressing step.
A useful technique for preparing vitrified grinding tools is hot pressing, which usually involves the simultaneous application of heat and pressure to the shaped material in a die. This technique can advantageously be used to obtain a very dense vitrified material at comparatively low molding pressures, e.g., 0.7 to 1.5 tons per square inch (tsi). While the resulting product often has a long working life, it may be deficient in some respects. For example, the product is limited to one grade of grinding ability or hardness, i.e., a hard grade characteristic of very low porosity (e.g., 0% to 5%). As a consequence, the product is "hard acting", i.e., its cutting surface will not break down readily. The hard-acting characteristic can unfortunately lead to unsuitable grinding, since the abrasive particles tend to dull and stop cutting; and the wheel faces tend to load. Furthermore, because of its relatively low viscosity, the glass portion of the dense, vitrified product may collapse under the pressure and temperature conditions utilized in hot pressing.
Efforts to reduce the density of abrasive materials by way of porosity inducement have been undertaken in the past. As an example, U.S. Pat. No. 1,986,850 of Pohl et al describes grinding bodies having a cellular structure, in which porosity is achieved via the formation of gasses when constituents within the body react with each other. In such a process, however, porosity of controlled size and distribution is often difficult to obtain.
U.S. Pat. No. 2,806,772 of Robie teaches the incorporation of thin-walled hollow spheres into phenolic-matrix abrasive materials. The spheres may be made from clay or various resins and plastics. The Robie invention appears to rely on cold pressing techniques, which often may not permit good control over the porosity and hardness of abrasive tools.
U.S. Pat. No. 2,986,455 of Sandmeyer also teaches the use of hollow spherical or globular abrasive particles to prepare porous grinding wheels. While Sandmeyer discloses hot pressing techniques, the reference does not appear to contemplate vitrified wheels in which porosity can be very accurately controlled over a wide range.
U.S. Pat. No. 4, 157,897 of Keat describes ceramic-bonded grinding tools which contain diamond or cubic boron nitride abrasive grits. The matrix bond includes either natural or synthetic graphite. Keat requires very low porosity in the matrix, i.e., less than 10%.
U.S. Pat. No. 4,799,939 of Bloecher et al describes abrasive products which include hollow glass bodies in an erodable matrix. The invention of Bloecher appears to be directed primarily to coated abrasives, and not to vitrified abrasive bodies such as cutting wheels.
U.S. Pat. No. 5,203,886 of Sheldon et al describes high porosity vitrified bonded grinding wheels, prepared by the use of bubbled alumina beads and particles of an organic pore-inducing material such as graphite, nut shells, or wood particles. Like Robie, however, Sheldon appears to rely on a cold-pressing technique, with its attendant disadvantages in some circumstances.
In view of the state of the art and some of the above-described drawbacks in that art, it is apparent that a need exists for hot-pressed, vitrified abrasive materials with improved grinding capabilities and adjustable grade characteristics. As a specific example, a need still exists for hot-pressed vitrified grinding wheels which are freer-cutting under a variety of working conditions.
It's also apparent that better methods for selectively varying the porosity (and consequently, the hardness characteristics) of hot-pressed vitrified materials need to be developed.